Facsimile synchronizing system



Aug. 27, 1940. w, (5 FlNcH 2212 968 FAGSIMILE SYNCHRONIZING SYSTEM FiledApril 21, 1938 I MODULATOR I AMPLIFER Q E 45 Y 719 TRAZEL ITFER Z3 o o-OSCILLATOR as 2 500 CYCLES '1 r? i I v \1 I k scANNER TONE CAM 44SYNCHRONIZING CAMS 45 RADIO RECEIVER RECT! FlER SYNCHRONiZ-I NG' M GNE 6A T SCANNER CLUTCH MOTOR 6Q INVENTOK. 'LU'i Ilia r11 8. 31! 3 i11ckATTORNEYS Patented Aug. 27, 1940 UNITED STATES PATENT OFFICE 3 Claims.

This invention relates to facsimile systems, and more particularlyrelates to novel methods of and means for maintaining a transmitter andreceiver in synchronous relation.

In accordance with my present invention, I

contemplate novel means for accurately predeterminlng the duration oftransmission of a synchronizing signal within very close limits. In theprior art, a single cam was generally used to both open and close thesame switch member to control the beginning and ending points of thetransmitted synchronizing current. I have found that for thesynchronizing signalto be most effective in facsimile work, it should beof very short duration, lasting on the order of onefiftieth of thescanning cycle, namely about seven mechanical degrees. I Prior artunitary cam switch control has not been reliable and cannot effect suchsmall accurately determined synchronizing signals due to mechanicallagging and other similar difficulties. In accordance with my presentinvention, I employ two switches in circuit with the alternating currentsynchronizing signal. The switches are 3 successively actuated topredetermine the instantof starting and stopping of the transmission ofthe synchronizing current. At the receiver novel means are provided forselectively segregating the "synchronizing current from the picture 3mechanism and maintain the receiver scanner in synchronous movement withthe transmitter scanner.

Further, in accordance with my present invention, novel circuitalarrangements are provided for coupling an individual facsimile unit,with an ordinary radio receiver. This coupling is effected I or in asimplemanner. The coupling unit comprises I so ing signal.

Still another object of my present invention is to provide a novelarrangement for accurately predetermining the cyclic starting andstopping positions for the transmission of a synchronizing current.

signals for actuating the start-stop synchronizing" A further object ofmyv present invention is to provide'novel methods of and means forselec-. tively receiving a synchronizing signal for maintaining arecorder in synchronous operation Still a further object of my presentinvention is 5 to provide a novel coupling unit betweenanindividualfacsimile recorder and a standard broadcast radio receiver.

These and further objects of my present invention will become apparentin the followingde 10 scription taken in connection with the drawing, inwhich:

Figure 1 is a schematic diagram of a preferred facsimile transmitterincorporating the princi-' ples'of my present invention. 15

Figure 2 is "a schematic diagram of a preferred facsimile recorderoperated in accordance with the principles of my present invention.

Figure 3 is a modified arrangement for connecting a facsimile recorderto a radioreceiver. 2o

facsimile system, although other well-known scanning units may be used.The transmitter scanner unit consists essentially of a source of light,generating a beam focusedupon sheet I l through element Hi.

The beam or pencil of light is reflected from the record sheet andfocused throughtubular lens element l4 upon a photoelectric cell l5contained within the scanner housing l6. As is well known in thefacsimile art, variations in the shading of the recording sheet I Ialong the scanning region correspondingly variably affect the intensityof light impinged upon photoelectric cell I5. The photoelectric cell isconnected in circuit with the modulator stage for producing electricalsignals varying in accordance with, the elemental shadingof the scannedrecord sheet.

The modulator stage is preferably a screen-grid vacuum tube, the controlgrid of which is connected to the anode of the photoelectric cell IS. Abiasing control potentiometer ll is employed to control the intensitylevel of the photoelectric 'cell variations upon the output of themodulator. A 0 carrier frequency current is connected to the screen-gridelectrode 18 of the modulator stage through a coupling condenser unitIS. The combination of electrical actions in the modulator stagegenerate a picture modulated carrier frequency 'current which is coupledto the amplifier stage through coupling impedance elements 20.

The output of the amplifier stage is coupled to a suitable transformer2| to a schematically indicated transmitter 22 for radio transmission toa remote receiver stage. It is to be understood that diiferentcommunication channels than radio may be employed. The output impedanceof transformer 2| may, for example, be 500 ohms to match the standardcoupling impedance of speech input circuits employed in radio broadcaststages. A high impedance level indicator 23 such as a D. B. meter, isconnected across the output of transformer 2| for assisting in theinitial adjustments of the transmitter unit.

An electronic oscillator 25 is employed to generate and supply the 2000cycle carrier frequency or tone to the modulator stage. Oscillator 25essentially comprises a double triode electronic tube 26 coupled throughtransformer element 21 to generate the sustained current oscillations atthe. required frequency. The output winding 28 of the oscillator has oneterminal connected to ground potential and the other terminal connectedto two potentiometer element units 30 and 3|. Potentiometer 30 isconnected to the coupling condenser I9 for the modulator stage throughcam-switch members 3233. When switch 32 33 is closed the output ofoscillator 25 is directly applied to the screen grid I8. This connectionoccurs preferably during one hundred and eighty mechanical degrees ofeach rotation cycle of control shaft 35. Actuation of cam switch 36 isthrough cam 31 secured on control shaft 35.

Scanner l0 generates picture signals in its excursion from left to rightcorresponding to a 180 rotation of the control shaft 35. The scanner isreturned to its left end position during the remaining 180 rotation ofshaft during which time no picture carrier signals are transmitted inthe preferred arrangement. Carrier tone switch 32-33 is opened bysuitable design of cam 31 at this point.

During the return stroke (to the left) of the scanner Hi, the frequencyof the generated current of oscillator 25 is changed to that of thesynchronizing frequency. In a preferred example, a 500 cyclesynchronizing current is used.

The frequency of oscillator 25 may be changed by shunting a condenserelement 38 between grid element 39 and ground. This switching iseffected by elements 40-4| of cam-switch 35 and occurs alternately withthe removal of carrier current from screen grid l8. By properlyproportioning the value of the capacitance of condenser 38, an accuratevalue of the 500 cycle generation is effected. It is to be understoodthat other known expedients for changing an oscillator frequency may beemployed for changing the normal picture carrier frequency of oscillator25 during. the return stroke of scanner l0 and that this frequency mightbe an increased value rather than a decreased one.

Potentiometer 3|, being in circuit with the output of oscillator 25, isdirectly connected to the secondary side of the output transformer 2| atpoint 46 through a novel arrangement for accurately predetermining theduration of the connection of the synchronizing signal across thispoint. In accordance with my invention, I provide a novel arrangementusing two switches 42 and 43 connected in series, respectively actuatedby cams 44 and 45. Switch 43 is in normally closed circuit relation;switch 44, in normally open circuit relation. No synchronizing currentchronizing signal transmissions per minute.

accordingly flows from potentiometer 3| to point 45 during the returnstroke period of scanner until switch 44 is closed.

Cams 44 and 45 are designed with short projecting nibs for actuating theassociated switches 42 and 43. Cam 44 is arranged on shaft 35 to closeswitch 42 at the predetermined instant the synchronizing signal is to bestarted for transmission during each cycle. A cam can be depended uponto accurately perform this closing function. The closure of switch 42completes the circuit between potentiometer 3| and point 45. Closure ofswitch 42 is made to preferably occur when scanner ID has practicallycompleted its return stroke.

It will be recalled that during this interval, switch 40-4| is closedand the output of oscillator 25 is at the synchronizing signalfrequency,

namely 500 cycles in the present example. The end point of thesynchronizing signal transmission is accurately determined by cam 45which is adjusted to actuate normally-closed switch 43 to open theswitch at the proper instant. It will now be evident that an accuratecontrol of the starting and stopping point of the synchronizing s gnalin each cycle can be accurately prearranged by my novel means.

The advantages of this arrangement by switches 42 and 43 are apparentwhen synchronizing signals of very small duration are required. I havefound through extensive experimentation that synchronizing signals ofshort duration more accurately effect synchronizing of the receiverscanner. Thus, when the cycle of the scanner is one second long, givingsixty lines of scanning per minute, I have found that a synchronizingsignal duration of one-fiftieth of this cycle, namely two-hundredths ofa second, is most satisfactory. My experiments have further shown thatthe angular position and duration of the synchronizing signal of thisshort duration and of even shorter duration can be readily accomplishedby the herein disclosed multiple switching means.

The synchronizing signal is ended just before the scanner l0 approachesits extreme left position in readiness for its picture scanningexcursion. At this time, cam 31 alters the position of cam switch 35,opening blades 4||4| to cause a picture tone of 2000 cycles to begenerated, and closing plates 32-33 to connect the 2000 cycle tone tothe screen grid I8 of the modulator stage. Control shaft 35-is rotatedat a predetermined speed such as by a synchronous motorthrough reductiongearing. In the given example, shaft 35 rotates at 60 revolutions perminute, effecting sixty scanning operations and corresponding syn- Thepicture modulated carrier and synchronizing signals are accordinglyalternately transmitted across radio transmitter channel 22 to theremote receiving stage.

' Figure 2 152. diagrammatic representation of a preferred system forrecording the transmitted facsimile signals. A standard broadcast radioreceiver is indicated at 50. The output stage 5| represents theconventional audio-frequency output amplifier stage coupled to loudspeaker 52 nal connected to ground and the other terminal connected tothe radio receiver through a coupling condenser 59. Switch 60 connectstransformer 56 to terminal 61 of the radio receiver. A preferredcoupling point to the radio receiver is to the anode 62 of the outputstage 51 thereof. This connection insures the maximum voltage nal outputfrom the radio receiver to the facsimile coupling unit 55.

Connecting-switch 60 may be manually operated when facsimile receptionis to commence. However, it is at present usual to connect the facsimilereceiver at the earliest morning hours since the broadcasting offacsimile programs occur at that time. Accordingly, switch 6| togetherwith the associated switches for starting both the radio receiver 50 andthe facsimile system in operation may be time-clock controlled in amanner well known in the art. A second switch 63 is shown mechanicallyganged with switch 60 to open the circuit connection to the loud speaker52 to prevent the facsimile signals from causing a sound disturbance. Itis entirely feasible to maintain the loud speaker 52 in circuit toaurally determine the quality and nature of the signals.

Coupling unit also contains a step-down power transformer 64 used toenergize the heater element of rectifier 51. Rectifier 51 is preferablyof the duo-diode type, with the anodes connected to the outer ends ofthe secondaryof coupling transformer 56. The 2000 cycle modulatedcarrier picture signal currents received by unit 50 are furtheramplified in magnitude by transformer 56 and then rectified at 51. Thesignal energy output is represented between the cathode 65 and ground.Cathode 65 is connected to the scanner 10 through switch blades 6|-68through the oscillating scanner arm 69. Arm 69 is electrically connectedto the pivotally mounted stylus point H which bears against record sheet12. Sheet 12 is secured in position against a metallic platen l3. PlatenI3 is electrically connected to ground to complete the picture signalcircuit to rectifier 51.

The diagrammatically illustrated recorder is similar to that describedin my co-pending application S. N. 156,625, filed July 31, 1937 entitledFacsimile recorder." Record sheet 12 is maintained in position toestablish contact with platen 13 and cooperate with the oscillatingstylus H. Record sheet 12 is a conducting paper which is electricallysensitized in accordance with i I l the picture signals to build up afacsimile reproduction corresponding to that transmitted from scannerl0. Heart-shaped cam 14, in cooperation with spring 15, produces anoscillating movinent of arm 69 about pivot 16.

A non-synchronous motor 11 is preferably employed to drive cam 14through a reduction gear box 18 and a friction clutch 19. Motor I1 maybe an A. C. induction motor, or a motor of the A. C.-D. C. type. Anelectrical governor is mechanically connected to the rotor of motor 11,and employed to maintain motor 11 at a predetermined speed of rotation.Governor 80 electrically opens and closes the 110 volt power leads tothe motor 11 to drive it at proper speed. A spark filter 8| is connectedacross governor 80 to minimize sparking.

Synchronous scanning movement of oscillating arm 69 is effected by meansof the governor controlled motor 1'! and the start-stop synchronousactuation of the clutch I9. Synchronizing magnet 82 is controlled by theshort duration synchronizing signal in a manner to be described. I

izing frequency of 500 cycles.

the scanner arm 69 will arrive at its initial leftend position inreadiness for its recording stroke,

. in suflicient time before reception of the synchronizing impulse tostart it off again for most operating conditions. Drivenplate 86 ofclutch 19 contains a projection 8'! which abuts armature 88 to stop therotation of shaft until the synchronizing impulse is received to releasearmature 88.

Cam switch 90 is maintained in the position illustrated during thereturn stroke of oscillator 69. Signal plate 61 is connected to the synchronizing magnet 82 through contact 9|. Normally no signal is impressedupon magnet 82 until a synchronizing impulse is received. I have foundthat the synchronizing impulse should be of very short duration toinitiate the scanning excursion at recorder NJ at the predeterminedstarting position. I have further found that the time lag for theoperation of the synchronizing magnet through its armature 88 to releasethe clutch I9 is, for recorders of the type described, of r mum limit isthat of the time lag for the synchronizing mechanism to function. At theinstant the synchronizing magnet is energized, cam

92 operates cam switch 90 to the right to open the synchronizing magnetcircuit and close signal.

blade 61 upon contact68, thereby impressing the v rectified picturesignals upon the stylus for recording on sheet12 in a manner alreadydescribed.

The recording stroke occurs during 180 mechanical degrees of rotation ofcontrol shaft 85 at which time the cam switch 90 is released to theillustrated position to repeat the cycle of operations described. 1

An important feature of my invention resides in the novel means forselectively controlling the impression of the synchronizing impulse uponmagnet 82 to avoid conflict between the synchronizing signal and picturesignal at theoutput of the rectifier, I provide a filtering meansembodied by condenser 93 to by-pass essentially all the signalfrequencies above the synchron- The 2000 cycle carrier together with itsside bands of picture signal modulations are well above the 500 cyclevalue so thatthe by-pass condenser 93 connected to one side of thesecondary of the coupling transformer 56 is usually sufliclent for suchcases. Condenser 93 by-passes the higher frequencies from transformer 56to ground through switch elements 9495. It is to be understood that myinvention contemplates more involved filtering means actuated by the camswitch 90 if more selective signal separation is required.

The preferable signal level for operating the recorder 10 is in theorder of to 300 volts. The expedient of the filtering condenser 93 hasvb found to readily reduce the 2000 cycle signal level toa value of 15%of the original and thereby insure the prevention of interference withthe 500 cycle tone passing through to the rectifier 51 chronizingimpulse is rectified at 51 and is applied to the synchronizing magnetthrough switch An alternate method of connecting the couplingtransformerv 56 of the signal coupling unit 55 to the output stage of astandard radio receiver is illustrated in Figure 3. A push-pull outputamplifier stage containing the triodes 5| and 5| is illustrated asconnected to the push-pull transformer 53', normally coupling this stageto the loud speaker unit. 60 is the time clock operated switchcorresponding to the switch 60 in Figure 2, and connects plate 62 oftube 5| to one side of the primary 58 of the coupling unit. A couplingcondenser 59' is shown connected to the opposite plate 62" to preventthe passage of direct current to transformer 56, The impedance of theprimary, winding 58 is preferably of the order of the impedance of theprimary winding of transformer 53'.

It is to be understood that my present invention may be practised invarious modified forms and arrangements without departing from thespirit and scope thereof and accordingly I do not intend to be limitedexcept as set forth in the following claims.

I claim:

1. A facsimile transmitter comprising means for generating a carrierfrequency current; means for modulating said carrier current in accord-2. A facsimile transmitter comprising means for generating a carrierfrequency current; means for modulating said carrier current inaccordance with picture signals; means for changing the frequency ofsaid generated carrier current to produce a synchronizing frequencycurrent between successive picture signal transmission intervals; andmeans including circuit connections for transmitting said changedcarrier current for a predetermined interval as a receiver synchronizingsignal comprising a first and second switch, said second switchinitiating said synchronizing signal and said first switch terminatingsaid synchronizing signal.

.3. A facsimile transmitter comprising an oscillator circuit forgenerating a carrier frequency current; means for modulating saidcarrier current in accordance with picture signals; means for reducingthe frequency of said generated carrier current to produce asynchronizing frequency current between successive picture signaltransmission intervals; and means including circuit connections from theoutput of said oscillator circuit for transmitting said reduced carriercurrent for a predetermined short interval as a receiver synchronizingsignal comprising first and second successively actuated switchesconnected in series relation, said first switch being in normally closedcircuit position, said second switch being in normally open circuitposition, said second switch initiating said synchronizing signal-WILLIAM G. H. FINCH.

